Evaluating the agronomic efficiency of alternative phosphorus sources applied in Brazilian tropical soils

Understanding the efficacy of alternative phosphorus (P) sources in tropical soils is crucial for sustainable farming, addressing resource constraints, mitigating environmental impact, improving crop productivity, and optimizing soil-specific solutions. While the topic holds great importance, current literature falls short in providing thorough, region-specific studies on the effectiveness of alternative P sources in Brazilian tropical soils for maize cultivation. Our aim was to assess the agronomic efficiency of alternative P sources concerning maize crop (Zea mays L.) attributes, including height, shoot dry weight, stem diameter, and nutrient accumulation, across five Brazilian tropical soils. In greenhouse conditions, we carried out a randomized complete block design, investigating two factors (soil type and P sources), evaluating five tropical soils with varying clay contents and three alternative sources of P, as well as a commercial source and a control group. We evaluated maize crop attributes such as height, dry weight biomass, and nutrient accumulation, P availability and agronomic efficiency. Our results showed that, although triple superphosphate (TSP) exhibited greater values than alternative P sources (precipitated phosphorus 1, precipitated phosphorus 2 and reactive phosphate) for maize crop attributes (e.g., height, stem diameter, shoot dry weight and phosphorus, nitrogen, sulfur, calcium and magnesium accumulation). For instance, PP1 source increased nutrient accumulation for phosphorus (P), nitrogen (N), and sulfur (S) by 37.05% and 75.98% (P), 34.39% and 72.07% (N), and 41.94% and 72.69% (S) in comparison to PP2 and RP, respectively. Additionally, PP1 substantially increased P availability in soils with high clay contents 15 days after planting (DAP), showing increases of 61.90%, 99.04%, and 38.09% greater than PP2, RP, and TSP. For Ca and Mg accumulation, the highest values were found in the COxisol2 soil when PP2 was applied, Ca = 44.31% and 69.48%; and Mg = 46.23 and 75.79%, greater than PP1 and RP, respectively. Finally, the highest values for relative agronomic efficiency were observed in COxisol2 when PP1 was applied. The precipitated phosphate sources (PP1 and PP2) exhibited a similar behavior to that of the commercial source (TSP), suggesting their potential use to reduce reliance on TSP fertilization, especially in soils with low clay contents. This study emphasized strategies for soil P management, aimed at assisting farmers in enhancing maize crop productivity while simultaneously addressing the effectiveness of alternative P sources of reduced costs.


Material and methods
The experiment was devised to compare a conventional, highly water-soluble phosphate fertilizer (TSP), against two distinct P sources derived from the phosphate fertilizer manufacturing process, alongside a sedimentary phosphate rock.These materials were applied in five soil types, varying P levels and clay contents (Sandy loam Entisol; Loamy sand Entisol; Clayey Oxisol 1; Sandy clay loam Oxisol; Clayey Oxisol 2) 15 .

Soil chemical attributes
Soil samples were collected from two farms, Lageado (Botucatu) and São Manoel, located in the state of São Paulo, Brazil.The geographical coordinates of these farms are approximately 22° 50′19″ S, 48°25′54″ W, at an elevation of 738 m.a.s.l, and 22°44′28′′ S, 48°34′37′′ W, at an elevation of 740 m.a.s.l, respectively.Before the implementation of the experiment, the studied five soils were submitted to chemical and particle size analysis.The soil chemical and particle size attributes are presented in Table 1.
For initial screening of soil chemical and particle size attributes, samples were collected at a depth of 0.20 m.Soil attributes, including pH, soil organic matter (OM) content, available P, K + , Ca 2+ , Mg 2+ , sum of bases (SB), cation exchange capacity (CEC), and base saturation (BS%), were determined.Soil pH was measured using a CaCl 2 0.01 mol L −1 solution 16 .Soil OM was determined using the rapid dichromatic oxidation method 17 .Soil exchangeable cations (available P, K + , Ca 2+ , and Mg 2+ ) were extracted using ion-exchange using ion exchange resin in a soil:resin:water (1:1:10, v/v).The samples were shaken for 16 h on an orbital shaker at 220 oscillations min -1 .The P concentration of the extract was performed by phosphomolybdenum blue complex formation in Table 1.Chemical and physical soil properties of the five soils varying clay contents before to start the greenhouse experiment.SB sum of bases, CEC cation exchange capacity, BS base saturation, PMAC phosphorus maximum adsorption capacity.SL Entisol sandy loam Entisol, LS Entisol loamy sand Entisol, C Oxisol 1 clayey Oxisol, SCL Oxisol sandy clay loam Oxisol, C Oxisol 2 clayey Oxisol.www.nature.com/scientificreports/sulfuric medium and with ascorbic acid as reducing agent, using a spectrophotometer.The exchangeable Ca, Mg, and K concentrations were determined by absorption spectrophotometry 16 .Also, the potential acidity (H + Al) was determined using calcium acetate 16 .The SB, cation exchange capacity (CEC), and base saturation (BS%) were determined following Raij et al. (2001).For the particle size analysis was determined by Claessen 18 .The P maximum adsorption capacity of the five studied soils was determined by shaking soil samples in a 0.01 mol L −1 CaCl 2 solution at a ratio of 1:10 (soil: solution) for 24 h, with varying concentrations of P ranging from 0 to 200 mg L −1 .The samples were then centrifuged at 3000 rpm for 15 min, and the remaining P in the solution was quantified using molybdate blue and ascorbic acid 19 .Finally, the P maximum adsorption capacities were determined using Langmuir's Isotherms 20 .

Chemical assessment of P fertilizers
To evaluate the agronomic efficiency of the examined phosphorus (P) fertilizers, we opted for three alternative P sources.Among these, two were derived from distinct stages of triple superphosphate (TSP) manufacturing (PP1 and PP2).PP1 originated from the production process of phosphoric acid, involving the precipitation reaction of dilute H 3 PO 4 with lime or calcium hydroxide.This process led to sedimentation, forming a dried powder 10 .On the other hand, PP2 was obtained through a physico-chemical reaction involving phosphate, iron, and aluminum salts.This reaction resulted in the formation of positively charged metallic ions, which subsequently reacted with negatively charged phosphate ions.The outcome was the creation of fine flakes that underwent precipitation during a flocculation phase; the products are manufactured in Minas Gerais, Brazil.The third source is reactive phosphate (RP) from Sechura desert region of Peru.This source derived from the decomposition of marine residues.These alternatives were then compared with a reference P source, triple superphosphate-TSP.The chemical and physical properties of the P sources were characterized in accordance with MAPA 21 (Table 2).

Experimental design
In a greenhouse experiment, we analyzed the effects of three alternative P sources on the agronomic attributes of Zea mays L., cultivated at five soil types.In greenhouse conditions, we carried out a randomized complete block design, investigating two factors (soil type content and P sources), evaluating five tropical soils with varying clay contents and three alternative sources of P, as well as a commercial source and a control group with four replicates (n = 4).Each plastic pot had a volume of 19 dm 3 , representing one experimental unit, totaling one hundred experimental units (Fig. 1).

Greenhouse experiment
Before the experiments began, the soils were sieved through a 4 mm mesh to remove roots, straw and clods and were then placed in the pots.The initial soil pH was adjusted by BS% method to meet the requirements of the maize crop (BS% = 70%) using dolomitic limestone (Relative Neutralizing Value = 98%).The soils remained properly moistened (70% of the field capacity-FC) to allow for limestone reaction, according to Raij et al. 22 .After 30 days of limestone application, five maize plants were cultivated in plastic pots, remaining two plants per pot, each containing 19 dm −3 of soil.Each experimental unit was fertilized with 120 mg P dm −3 (furrow application), 120 mg K dm −3 and 60 mg N dm −3 .The nitrogen (N) and potassium (K) fertilization were divided into two applications: at 15 and 30 DAP emergence.Additionally, micronutrients (B, Zn, Cu, Fe, and Mn) were applied via soil in silicate oxide form in each plot as recommended by Raij et al. 22 .All fertilizers were applied based on initial nutrient content on soil and meeting crop requirements for 10-12 t ha −1 yield goal.The pots were watered daily and maintained at 70% of FC, with water levels monitored by weighing.

Plant morphological attributes and nutrient accumulation
Plant measurements were taken on 55 DAP emergence, including: (i) stem diameter (at 1 cm above the soil; (ii) plant height; (iii) shoot dry weight; (iv) accumulated nutrients in shoot.The shoot dry weight (in grams) was determined by drying the samples for 48 h at 65 °C.To determine the nutrients accumulated in the shoot dry weight, the samples were ground in a knife mill and passed through a 2-mm sieve.To determine the nutrients content in the shoot dry weight, we used the methodology described by Malavolta et al. 23 .The nutrient accumulation was calculated as follows: Nutrient accumulation (g kg −1 ) = dry weight (g plant -1 ) × nutrient content (g kg −1 ) 24 .www.nature.com/scientificreports/

Soluble phosphorus
The soluble P content was determined twice during the greenhouse experiment (15 and 30 DAP emergence) and followed the porous capsules extractor method 25 .Initially, ceramic porous capsules (6 cm height × 19 mm diameter) were installed at a depth of 10 cm.Then, a tension of 70 kPa was applied through a vacuum pump, and the extracted soil solution was packed in the plastic pots.After, the P contents in the soil solution was determined by colorimetry according to Raij et al. 16 .

Relative agronomic efficiency
Relative agronomic efficiency (RAE) was calculated by dividing each P source treatment to the reference fertilizer (TSP) added at the same fertilization dose, following the equation proposed by Büll et al. 26 : where RAE (%) is the relative agronomic efficiency, YaP is dry matter production by plants in a given alternative P fertilizer (mg plant −1 ); YrP is the dry matter production by plants in the reference P fertilizer (TSP).

Statistical analysis
Prior to data analysis, all dataset was tested for normality with Shapiro-Wilk's test ("shapiro.test"function), next all variables were analyzed with a two-way ANOVA with the main factor the P source used, the soil type as secondary factor, and pots number as a random factor.Tukey's test was used as the post hoc test (p < 0.05).We performed principal component analysis (PCA) to outline the relationship between soil type and plant nutrient contents.All statistical analyses were performed using the packages Hmisc, psych, car, ade4, and vegan using R core Team 27 .

Experimental research and field studies on plants including the collection of plant material
The authors declare that the cultivation of plants and carrying out study in São Paulo State University (UNESP), complies with all relevant institutional, national and international guidelines and treaties.

Plant height, stem diameter and shoot dry weight
The plant height, stem diameter, and shoot dry weight were affected differently depending on the fertilizer (Table 3).The TSP source showed the highest values (e.g., plant height, stem diameter, and shoot dry biomass) in all studied soils.On the other hand, among alternative P sources, PP1 showed to be the most efficient one, increasing the plant height of up to 15.87% and 54.96% in comparison to PP2 and RP sources in the C Oxisol 2 , respectively.The PP2 source increased stem diameter by up to 14.68% and 47.25% higher than PP1 and PR, respectively.The PP1 source increased the shoot dry weight of up to 41.24% and 73.41% when compared to PP1 and PR.

Phosphorus in soil solution
A pronounced difference was found for soil solution P among the tropical soils when P sources were applied.Initially, at 15 DAE, the highest values were found in the C Oxisol 1 when PP1 source was applied (Fig. 2A).In this soil, the soluble P was 61.90%, 99.04%, and 38.09% higher than PP2, RP and TSP, respectively.After 30 days, TSP presented greater values in SL Entisol, LS Entisol and CO xisol 1 .Also, those alternative sources (PP1 and PP2) showed the highest soluble P values in the SCL Oxisol and C Oxisol 2 (Fig. 2B).

Nutrients accumulation in the shoot
The two-way ANOVA showed significant differences among the P sources for the nutrients' accumulation in the shoots (Table 4).The highest values for P, N, S, Ca, and Mg were observed where TSP was applied, to all studied soils.However, when evaluating only alternative P sources, the highest values for P, N, and S accumulation were greater in the PP1, showing an increase of up to 37.05% and 75.98% (P), 34.39% and 72.07%(N); and 41.94% and 72.69% (S) in comparison to PP2 and RP, respectively.Among the studied soils, our results showed that maize plants cultivated in the SL Entisol, LS Entisol, and C Oxisol 2 had the highest accumulated P in the shoots.For Ca and Mg, the highest values were found in the SCL Oxisol when PP2 was applied -44.31% and 69.48% for Ca and 46.23 and 75.79% for Mg, respectively, in comparison to PP1 and RP.It was not observed a significant response for K accumulation.When the soil types were compared, the highest values of accumulated K were found in the C Oxisol 2 .On the other hand, when comparing the P sources, the highest values were found for TSP followed by PP1 (Table 4).

Relative agronomic efficiency
Relative agronomic efficiency (RAE) of alternative P sources is shown in Fig. 3.The highest values were found in the C Oxisol 2 when PP1 was applied.This source showed an increase of 49.75% and 88.14% in comparison to PP2 and RP.

Multivariate analysis
The principal component analysis (PCA) showed that K, N, P, Ca, Mg, S accumulated in plant tissues, height, stem diameter, and shoot dry weight were the main factors contributing to the variance of the samples (Fig. 4).
The first two principal components (PC1 and PC2) accounting for 75.43% of the data variance were assumed as the most principal components.The analysis showed the following aspects: (i) a positive correlation between N and K accumulation; (ii) positive correlation between Ca and Mg accumulation; (iii) positive correlation between stem diameter and shoot dry weight (Fig. 4).

Discussion
The results highlighted the effects of alternative P sources on maize crop nutrition and shoot biomass production in various Brazilian tropical soils.We aimed to understand how these sources enhance nutrient accumulation and morphological attributes in maize crops through a greenhouse experiment.The findings indicated that alternative P sources from manufactured fertilizers can increase soil P levels and nutrient uptake by maize plants in soils with www.nature.com/scientificreports/varying clay content.Our results revealed significant differences among the studied P sources in terms of plant attributes (e.g., height, stem diameter, and shoot dry weight) and nutrient accumulation in plant tissues as well.
Several studies have emphasized the role of alternative P sources as a sustainable approach, primarily due to their slow release and positive impact on crop growth [28][29][30] .The increase in shoot dry weight is directly linked to P availability for crops associated with improved photosynthetic activity, including Rubisco activity and regeneration.On the other hand, low shoot dry weight can be attributed to a reduction in the maximum quantum yield related to the photochemical system, which occurs under conditions of limited P availability 31 .
The P use efficiency (PUE) in tropical soils is very low and it is mainly linked to soil chemical properties such as PMAC, clay content, and soil pH.These factors play a crucial role in determining P availability because they are closely associated with the adsorption process 6 .In our study, we observed that the PP1 (precipitated phosphate 1) and PP2 (precipitated phosphate 2) sources exhibited slightly lower plant morphological and nutritional attributes, and agronomic efficiency when compared to conventional sources like TSP.This finding supports our hypothesis that alternative sources can enhance P availability for crops, potentially reducing reliance on TSP application in tropical conditions.Notably, our results indicated that even when PMAC from www.nature.com/scientificreports/113.51 to 1015.4 mg kg −1 , the alternative sources performed similarly to TSP.This similarity can be attributed to the slow release of these alternative sources, which may result in increased P availability for crops during periods of high demand 32 .Some studies have described that alternative phosphorus sources (e.g., precipitated phosphate) could exhibit a residual effect.For instance, studies by Leal et al. 10 , evaluating alternative phosphorus sources from the manufacturing of TSP, showed an increase in biomass production of grasses under acidic conditions, and after successive cuts, the residual effect showed greater values when compared to TSP. Results obtained by Bogdan et al. 33 showed that alternative phosphorus sources can gradually release phosphorus for up to seven months, demonstrating higher availability compared to TSP by up to 42% during the growth of perennial ryegrass.Ashekuzzaman et al. 34 demonstrated that the application of precipitated phosphorus sources derived from industrial processes can exhibit a residual effect and increase phosphorus bioavailability by up to 109%.
The stability of the soil solution during the sampling period (e.g., 15 and 30 days) in a clayey soil ( C Oxisol 2 ) is linked to soil physical-chemical properties, including soil organic matter (SOM), PMAC, clay content, and soil pH.According to Wang et al. 35 , the rise in soil pH (e.g., 5.5-6.5)critically influences phosphorus (P) pools in soils.This factor regulates the solubility and distribution of minerals, altering adsorption dynamics in clayey soils.In studies by Sandim et al. 14 , the PP1 source increased the moderately labile P pools, extracted with HCl and formed by Ca-P.Calcium (Ca) presence may result from lime application or be inherent in the source composition.
Studies by Wang et al. 35 demonstrated that root activity governs phosphorus solubility, availability, and pools, influencing soil microbial activity through root exudation.Finally, root activity (e.g., organic acid exudation) can chelate Ca present in calcium-phosphate bonds (e.g., through hydroxyl and carboxyl groups) or compete with phosphate for adsorption sites, thereby increasing phosphorus solubility 36 .According to Veloso et al. 37 , in clayey Oxisol, the modest stress (e.g., low phosphorus release at the time) can increase enzymatic activity (e.g., acid phosphatase) by plants, microbial activity, or interactions between them.Thus, another possible explanation for greater values of PP1 and PP2 in C Oxisol 2 can be a soil organic matter content supporting microbial activity and increasing enzymatic activity or the positive feedback between plant and soil biota, increasing P release of alternative P sources.
This finding helps explain the greater biomass production observed when using this source (Table 3), as this P fraction remains in equilibrium with the soil solution, making it readily available to crops during their development.Several studies [38][39][40] have shown that alternative P sources can enhance plant growth and biomass production.This effect can be attributed to changes in the plant-soil continuum, influencing P dynamics in the plant rhizosphere through three main mechanisms: (i) Plants can modify their root environment by extruding H + ions, leading to rhizosphere acidification, which enhances the solubilization of P from these sources 41 ; (ii) Root exudates stimulate microbial activity, including enzymes like phytase and acid phosphatase, especially in low P conditions; (iii) The symbiosis association with arbuscular mycorrhizal fungi (AMF), increasing plant growth and improving soil structure and nutrient contents (mainly P) thus increasing plant productivity 42 .
However, as described by Ai et al. 43 , root exudates may promote microbial metabolism without increasing microbial biomass P utilization.This implies that there may not be competition between microorganisms and plant roots for P in P-poor soils or when slow-release P sources are applied.Furthermore, Rezakhani et al. 44 demonstrated that low-solubility P fertilizers can boost shoot dry weight by stimulating solubilizing microorganisms, which not only enhance P availability but also influence the uptake of other elements like silicon (Si).This, in turn, promotes increased shoot and root biomass, creating a positive feedback loop in the rhizosphere.
The P application resulted in increased nutrient accumulation, particularly when alternative P sources were used, leading to higher levels of N, S, and Mg in plant tissues when compared to TSP.It is well-established that P content in plants is closely associated with the uptake and accumulation of other nutrients 45 .In fact, P supplementation has been shown to enhance nutrient content in maize crops 3 .Alternative P sources have the capacity to release nutrients that are part of their composition, often facilitated by microbial solubilization.Additionally, P plays a vital role in plant physiology by contributing to the ATP pools, providing the energy needed for nutrient uptake and biomass production.The application of P fertilizers can also expose plant roots to a larger surface area, thereby increasing nutrient absorption.Consequently, the observed increases in key nutrients such as N and Mg are directly linked to photosynthetic processes and overall biomass production 46 .
The nutrient accumulation was most pronounced when maize crops were cultivated in soils characterized by low and mean PMAC levels.Recent reports have described that higher clay contents may contribute to SOM stabilization in soils [47][48][49] .Recently, Liu et al. 49 , found that hydroxy-interlayered clay minerals, creating a hydroxy-Al polymer with organic particles, reduced the oxidation.Here, we found that C Oxisol 2 showed a similar clay content when compared to C Oxisol 1 , however, with much greater SOM content (Table 1).This observation suggests that the buffer capacity of tropical soils may influence the nutrient accumulation.Specifically, sandy and medium texture soils and clayey soils with high SOM ( SL Entisol, SCL Oxisol, and C Oxisol 2 ) sources led to increases in both nutrient content and shoot dry weight production (as showed in PCA).In soils with higher PMAC values ( C Oxisol 1 ), the P buffer capacity may become a limiting factor for production.According to Yang et al. 50, the SOM dynamics can regulate P adsorption/desorption and control its availability under certain conditions.Notably, crops with rapid growth cycles like maize tend to be more productive in loam and sandy soils than in clayey soils 51 .Therefore, optimizing PUE under these conditions is paramount, with considerations for slow-release fertilizers and alternative P sources.
The PCA provided valuable insights into the influence of soil texture on nutrient accumulation and shoot dry weight production under tropical conditions, elucidating the intricate relationship between soil type and key plant parameters, including nutrient uptake.We observed that soils with a moderate clay content may exhibit a harmonious equilibrium in their charge properties, thereby facilitating heightened nutrient accumulation in plant shoots, particularly P, K, and N. Conversely, sandy soils ( SL Entisol) showed similar correlations with plant attributes, possibly due to their lower adsorption capacity (lower surface charges than clayey soils) and an increased concentration of nutrients in the soil solution without losses via leaching (Fig. 4).This resulted in a significant accumulation of these nutrients.These findings agree with studies that report the importance of accounting for soil texture when considering P fertilization, plant attributes, and nutrient accumulation strategies 52 .
Our greenhouse experiment showed which alternative P source had potential to enhance plant height, shoot dry weight, stem diameter, and nutrient accumulation.As far as we know, this is the first report on the agronomic efficiency of by-products from phosphate fertilizer production in Brazilian tropical soils.These results could be of great importance for the valorization of these by-products in agriculture, reducing costs associated with commercial sources, increasing productivity, and reducing Brazil's international dependence on phosphate fertilizers.Finally, further studies should be conducted to understand how the application of alternative P sources can influence the rhizosphere microbiome and increase P availability.

Table 2 .
Chemical characterization of P sources used in the greenhouse experiment.TP 2 O 5 total phosphate, P 2 O 5 CA citric acid, P 2 O 5 NAC neutral ammonium citrate, W water. Methods applied were performed according to the MAPA (2017).PP1 precipitated P, PP2 precipitated P, RP reactive phosphate rock, TSP triple superphosphate.

Table 4 .
Nutrients accumulation (mean ± SD) in shoot biomass of maize crops cultivated in five soils, after application of alternative P sources.Different lowercase letters within rows differ by Tukey's test at α = 0.05, and different capital letters within columns differ by the same post-hoc test.Numbers represent the mean values (n = 4) followed by the standard deviation (SD).SL Entisol sandy loam Entisol, LS Entisol loamy sand Entisol.C Oxisol 1 clayey Oxisol, SCL Oxisol sandy clay loam Oxisol, C Oxisol 2 clayey Oxisol.PP1 precipitated P, PP2 precipitated P, RP reactive phosphate rock, TSP triple superphosphate.